In my copending Canadian application No. 383,217 filed on Aug. 5, 1981, which corresponds to my U.S. application Ser. No. 343,359, filed concurrently herewith there is disclosed a boom-mounted felling head which incorporates a horizontally oriented circular saw blade in the lower portion, and tree grappling means in the upper portion, in a construction which is such as to allow the saw blade to cut through the trunk of a tree before the tree grappling means is activated to clasp the tree. This results in the considerable advantage of leaving the tree trunk completely unstressed and without bending moments of any kind during the cutting phase. It will be appreciated that, if the design had been such that the tree grappling means were in engagement with the tree during the cutting procedure by the circular saw blade, any misalignment between the tree grappling means and the tree itself would place a localized bending moment on the tree trunk, so that as the saw blade were nearing the completion of its cut, it is likely that the tree, both above and below the location of the cut or kerf, would splinter. Any splintering results in an unusable end portion for the cut tree, and reduces the usable production capability of the machine.
The main embodiment disclosed in the said earlier Canadian patent application Ser. No. 383,217 is one in which the felling head is adapted for "straight-ahead" operation, in which the motion of the felling head during the cutting phase is intended to be along a line parallel with the boom.
There are two operations which can accomplish such straight-ahead cutting for a felling head of the kind just mentioned. In the first procedure, the boom is held stationary in front of the vehicle, and the vehicle is driven forwardly to carry the felling head toward a tree in the path of the vehicle. This is a somewhat uncertain maneuver because of the possibility of uneven terrain beneath the vehicle. In the case of uneven terrain, the boom and the felling head can be caused to oscillate or gyrate in such a way that it is difficult to arrive at a clean cut. The second operation is to keep the vehicle stationary and to extend the boom. In the case of a telescoping boom, the felling head would normally be controllable and capable of producing a clean cut. However, in the case of a knuckle boom, in which two boom portions are articulated together with the felling head being articulated to the end of the remote portion, a complex arrangement of hydraulic controls and compensating maneuvers must be used in order to allow the felling head to move forwardly without gradually tilting, i.e. without a shift in the orientation of the plane in which the circular cutting saw is rotating.